In percutaneous transluminal angioplasty procedures, a catheter having an expansible distal end, usually in the form of a balloon, is positioned in a lumen of a blood vessel with the distal end disposed within a stenotic atherosclerotic region of the vessel. The expansible end is then expanded to dilate the vessel and restore adequate blood flow through the diseased region.
Balloons on conventional angioplasty catheters have low compliance. Therefore, after the balloon is internally pressurized, a further increase of the internal pressure will cause the outside diameter of the balloon to increase only minimally. This prevents the balloon from overexpanding and causing substantial damage to the vessel.
Interventional cardiologists, use on average 1.5 balloon catheters for each procedure. The first catheter used is chosen to have the largest balloon that will cross the stenosis and will also have an inflated diameter of the order of the vessel diameter proximal to the stenosis. Often the diameter of this balloon is smaller than desired in order to cross the lesion. After the first catheter has been expanded, a second catheter is therefore often needed to further dilate the lesion. Typically, the increase in balloon dilatation diameter is 0.5 mm. For example, the first catheter may have a balloon having an expanded diameter of 2.5 mm. In a standard procedure, after the first balloon has been expanded, it is deflated, removed from the patient and a second balloon catheter, typically having a 3.0 mm diameter balloon when expanded, is introduced to the lesion. The balloon is then positioned at the region of stenosis and expanded to its full diameter to further dilate the vessel to the proper size. Since dilatation balloon catheters are designed for single use only and since they are expensive, presently about U.S. $300 to U.S. $800 each, the use of two balloon catheters for each procedure substantially increases the cost of the procedure. Use of two balloon catheters in sequence also increases the length of the procedure.
At times it is desired to deliver a therapeutic agent, typically a drug, to a target site through a body lumen such as an artery, vein or other hollow organs of the gastrointestinal, genitourologic or pulmonary systems. One way to deliver a drug to a target site along the wall of a body passage or lumen is disclosed in U.S. Pat. No. 5,304,121 to Sahatjian. It shows a dilatation balloon catheter in which the balloon is coated with a hydrogel polymer layer incorporating a drug in aqueous solution. Upon expansion of the balloon the drug is released to the wall of the body lumen at the target site. This method has its drawbacks and limitations. Hydrogel layers can contain only a limited amount of drug and the drug tends to diffuse quickly out of the hydrogel where it is subject to being washed away by the blood stream prior to reaching the target site. Both of these factors can limit the effectiveness of using a hydrogel layer to deliver a drug to a target site.